Fig 1.
Testing apparatus and experimental setup.
Left: Balans chair inside custom wood-built testing apparatus. In this setup, the pelvis is stabilized by securing padded arms (green arrows) around the anterolateral aspect of the pelvis and locking the pads into place. Right: Sagittal view of participant set up for testing with the pelvis locked into place. The tension load cell (red arrow) measuring extensor force generated by the participant is secured to a chest harness at the height of the xiphoid process.
Fig 2.
Schematic of the study protocol and testing timeline.
Participants completed two testing sessions separated by 2–7 days. Within-day reliability data was calculated using the testing blocks in session 2. Between-day reliability as calculated from testing block 2 of each session.
Fig 3.
Force modulation feedback and time series.
Left: A recreation of visual feedback located on monitor 1m in front of the participant. The two horizontal lines representing the target force fluctuate up and down while the participant tries to keep their generated force (orange) between these two lines. Right: Time series plot of target force (black line) fluctuating between 20–50% maximum voluntary force and the participant-generated force (orange line). The first repetition (sine wave) and the ascending portion of the last repetition in each trial were eliminated to allow the required time to develop or drop below the lower force boundary (gray boxes).
Fig 4.
A. Example of muscle synergies that were extracted from a single subject’s time- and amplitude-normalized EMG during condition B (nine repetitions of task). Temporal patterns multiplied by weighting coefficients from the extracted synergies recreate the normalized muscle activations. B. Individual muscle activation is the sum of the contribution of each synergy. Y-scale of normalized EMG where 1 represents 100% MVIC. SYN, synergy; LTES, left thoracic erector spinae; RTES, right thoracic erector spinae; LLES, left lumbar erector spinae; RLES, right lumbar erector spinae; LLM, left lumbar multifidus; RLM, right lumbar multifidus; LGMX, left gluteus maximus; RGMX, right gluteus maximus; LEO, left external oblique; REO, right external oblique.
Table 1.
Characteristics of study participants.
Fig 5.
Force modulation accuracy of the individual subjects across conditions.
Each colored line represents an individual subject’s mean force modulation error (RMSE) from the testing blocks across the three conditions. Generally, error increased from protocol A to B and again from B to C. Faster rates of force modulation also resulted in greater between-subject variability. RMSE, root mean square error.
Table 2.
ICC, SEM, and MDD values for test-retest reliability of trunk extensor force modulation protocol.
Fig 6.
Bland Altman plots for within- and between-day comparisons of RMSE.
Limits of agreement (LOA) include 95% of differences between the two measurements. Systematic bias is observed where individuals who demonstrated higher mean RMSE values tended to have larger differences between measurements. This is most evident in condition A. RMSE: root mean square error, SD: standard deviation.
Table 3.
Mean rate of perceived exertion (RPE 6–20) for conditions A, B, and C across four testing blocks.
Fig 7.
Mean amplitude normalized sEMG activation (+ SD) at minimum (20% of MVF, blue) and maximum target forces (50% of MVF, gray) during the force modulation task.
EMG data were averaged across repetitions and participants. Right-sided muscles visually tended to demonstrate slightly more activation than left-sided muscles. MVF, maximal voluntary force; MVIC, maximal voluntary isometric contraction; LTES, left thoracic erector spinae; RTES, right thoracic erector spinae; LLES, left lumbar erector spinae; RLES, right lumbar erector spinae; LLM, left lumbar multifidus; RLM, right lumbar multifidus; LGMX, left gluteus maximus; RGMX, right gluteus maximus; LEO, left external oblique; REO, right external oblique.
Fig 8.
Mean time-series normalized muscle activation patterns during two modulation repetitions of each condition.
Each repetition represents 500 data points. Only right-sided muscles are presented in this figure (left-side muscles show a similar pattern). The right external oblique muscle (REO) is minimally active across conditions. All other muscles appear to contribute to the force modulation. MVF, maximal voluntary force; MVIC, maximal voluntary isometric contraction; RTES, right thoracic erector spinae; RLES, right lumbar erector spinae; RLM, right lumbar multifidus; RGMX, right gluteus maximus.
Fig 9.
Group averaged muscle synergies extracted by non-negative matrix factorization.
Temporal patterns and weighting coefficients (+SD) for three different conditions. Synergies presented in the same color between conditions indicate the same synergies with the mean scalar product greater than 0.85. The scalar products calculated compared to the synergies in condition A are presented on the graphs of conditions B and C, indicating the similarity between those synergies across conditions. SYN, synergy; LTES, left thoracic erector spinae; RTES, right thoracic erector spinae; LLES, left lumbar erector spinae; RLES, right lumbar erector spinae; LLM, left lumbar multifidus; RLM, right lumbar multifidus; LGMX, left gluteus maximus; RGMX, right gluteus maximus; LEO, left external oblique; REO, right external oblique.